A thermostat is a temperature activated valve used in internal combustion engines to regulate the flow of liquid coolant. When the thermostat valve is closed, the flow of liquid coolant is prevented from being circulated through a heat exchanger such as a radiator and so the engine is allowed to quickly heat up to its optimum or design operating temperature. The engine recirculation circuit is typically restricted to a small volume of fluid compared to the overall or total volume of the cooling system. When the thermostat valve is opened, coolant from the larger cold volume is permitted into the smaller volume engine recirculating system and passes through the cylinder head where it gets hot. It is then circulated from the engine into a radiator or other heat exchanger where it loses the heat to the air flowing through the radiator. The cooled fluid is then led back to the engine where it can be used to remove further heat from the engine, recirculated to the radiator and so on. A pump driven from the engine propels the coolant around the system.
This type of thermostat valve typically operates mechanically. Most commonly, a wax pellet is provided inside a sealed chamber. The wax is solid at low temperatures, but as the engine heats up the wax melts and expands. As the wax expands, it pushes an actuator rod outwardly from the chamber that in turn opens the valve. By altering the composition of the wax, the temperature at which the valve opens can be controlled. Typically, such a thermostat valves operate generally in the range of 60° C. to 100° C. and in certain cases may run cooler or even hotter.
To reduce emissions and pollution, modern engines are designed to run hotter than before, typically over 80° C. and even higher. This permits the engine to operate more efficiently as well. However, a higher operating temperature for the engine creates certain problems for the cooling system in general and the thermostat in particular. In particular, mechanical thermostats are mass-produced as economically as possible. They typically include a stabilizing spring to position the valve components away from the base. This is desirable, because the connection is flexible and will not fatigue and fail like a rigid or fixed joint. When such a device is closed, the parts are anchored somewhat by the contact between the valve and the valve seat. However, when the valve is lifted off the valve seat as the valve opens, then the components that are flexibly attached are freeer to move, leading to vibration and chatter of the valve due to engine vibration and the like. This results in an uncontrolled gap opening between the valve and the valve seat meaning an uncontrolled flow of coolant from the cold side to the warm side through the valve.
Once the valve opens, there is a rush of cold coolant that floods the engine cooling system and creates a dramatic temperature drop that is hard on the engine components, because of such a flexible or resilient joint. The cold fluid tends to rush in because the thermostat valves are operating in a boisterous environment. The pressures in the cooling system can be large forcing the flow somewhat. The engine itself is typically vibrating and shaking, leading to chatter of the valve as it opens. The valve can be knocked askew by the fluid movement, resulting in a larger opening than intended for that temperature. As a result the thermostat permits large and sudden flows of cold fluid into the engine block when the valve first opens.
Cold fluid in a hot engine is problematic. For example, this initial drastic change of temperature has been known to cause head gasket failure in certain vehicles. Essentially, the engine heats up quickly as compared to the rest of the system and then the thermostat opens. This permits a sudden, and given the relative volumes a rather sustained rush of cold fluid to enter the head causing a temperature shock to the engine components. This problem is most acute in colder climates on initial start-up, when the coolant outside of the engine recirculation circuit may be at a very low temperature as compared to the engine temperature, due to very cold ambient conditions.
In addition to being hard on the engine components, such a sudden rush of cold fluid can drastically reduce efficiency and performance of the engine during the duration of the cold flow. As a result, a sudden plume of pollution temporarily surges from the tailpipe or exhaust until the engine and coolant system attain an optimum balance. However, it can take some time for the heat of the engine to warm the larger volume of cold cooling liquid and until it does so the engine is being over cooled and is both inefficient and polluting.
The problem identified therefore is this sudden rush of coolant into the hot running engine of the vehicle as the valve first opens. One way to address this problem is to have an electronically controlled thermostat, where the degree of opening up thermostat, and thus volume of fluid flow can be precisely controlled. However, such thermostats are extremely expensive and not suitable for most mass production OEM type applications. Another alternative is to try to vary the composition of the wax pellet to cause differential displacement of the piston over the temperature activation range. However, this also is extremely difficult to attain in practice because the expansion of mixed composition wax is not easy to implement or control. It is very difficult, if not impossible to control the rate of opening between the open and closed positions through wax composition changes alone.
What is needed is a simple and efficient method for controlling the initial inflow of liquid coolant into a hot engine through a mechanically actuated thermostat valve as the valve opens. What is also desired is a simple and efficient structure and method, in such a flow controlled thermostat, of ensuring that the thermostat fails in an open or safe position, in the event of an overheating event.